Notes on EMC Standards
Zainal Salam: Power Electronics and Drives
NOTES ON EMC STANDARDS, ASPECTS AND SOLUTIONS
1.0
Present EMC Standards
There are standards that need to be complied for Electro Magnetic Compatibility – e.g. EU
336/89.
Figure 1 shows maximum limit of conducted Electro Magnetic Interference (EMI) set by
the FCC and VDE for equipment used in industrial, commercial and residential equipment
(not medical). The EMI noise frequency ranges from 10Khz to 30Mhz.
To compare against these limits, the conducted noise is measured by means of a specified
impedance network called LISN (Line Impedance Stabilisation Network). Standard for
radiated EMI are also specified by various agencies.
Figure 1
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Notes on EMC Standards
2.0
Zainal Salam: Power Electronics and Drives
Origins of high frequency EMI
Common sources of EMI in power electronics systems are:
•
•
•
•
•
2.0
Rectifying diodes,
Switching components (IGBTs)
High frequency transformers and chokes.
Circuit layouts (high dv/dt long wires, high dv/dt wide loops, high current wires)
Mechanical switches bouncing
Radiated and Conducted EMI/RFI
There are two forms of EMI/RFI propagation of interest as shown in Figure 2:
•
•
Electromagnetic radiated E and H waves.
Conducted Interference on supply lines and interconnecting wires.
Radiated interference is normally minimised, as a result of layout and wiring practices
required to reduce leakage inductance. Typically the high frequency current loops will be
short and twisted pairs will be used where possible. Transformers and chokes will be
screened to reduce radiated magnetic fields. Screen box or enclosure will often be used.
Furthermore the techniques applied to minimised conducted interference will also reduce
radiated noise.
Radiated high
frequency noise
Chassis/heatsink
Conducted Noise
Power
Source
Converter
Earth
Figure 2
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Notes on EMC Standards
Zainal Salam: Power Electronics and Drives
In general, the followings are observed:
•
•
•
•
•
4.0
EMI are propagated both as conducted and radiated
Noise currents flow through connecting wires
Loops and wires act like antennas producing radiated EMI
Loops and wires also receive EMI
Therefore, EMI are radiated as well as picked-up by loops and wires.
Powerline Conducted-mode Interference
Two major aspects of conducted interference will be considered:
4.1 Differential-mode interference (series mode)
Component of RF noise which exists between any two supply or output lines. In SMPS,
this would be live to neutral, line to line at the input. At the output, noise exists between
the positive and the negative lines.
4.2 Common Mode interference
Component of RF noise which exists on any or all supply or output lines with respect to
the common ground plane (chassis, box heatsink).
5.0 Propagation of EMI
EMI/RFI can propogate mainly by two ways:
•
Capacitive coupling
•
Inductive coupling
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Notes on EMC Standards
5.1
Zainal Salam: Power Electronics and Drives
Capacitive Coupling
Conductor A
Conductor B
Cstray
+
Vnoise
-
Z
Vac
Figure 3
•
•
•
Capacitive coupling between conductors causes parasitic currents
Noise voltage increase with frequency. Higher frequency means more high frequency
harmonics flow through the capacitor.
Two wires with 2mm diameter and spaced by 1cm shows about 0.1pF/cm of parasitic
capacitance.
5.2
Inductive Coupling
La
Iac
Conductor A
Conductor B
M
Lb
Vac
Vnoise
Inoise
Figure 4
•
•
•
Magnetic coupling between conductors causes parasitic induced voltages
Noise current increases with frequency.
Two wires with 2mm diameter and spaced by 1cm, shows about 10nH/cm of parasitic
inductance.
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Notes on EMC Standards
Zainal Salam: Power Electronics and Drives
6.0 Corrective Methods
•
•
•
•
Improved Circuit layouts – ground planes
Shielding of noisy components and wires
Position of magnetic components
EMI Filters
7.0 EMI Filters
7.1
Differential Mode Filters
Example: single-phase system.
Power
Source
XCAP
Diode Bridge
C5
Figure 5
7.1.1 X-CAP
Main harmonic filter is normally known as X-CAP. It should be rated for the switching
frequency and its harmonics, i.e. it will absorb all harmonics multiple of switching
frequencies.
Say switching frequency is 20KHz, then for 10-ohm reactance, we need X-CAP to be
0.8uF.
7.1.2
Additional X-CAPs (C5).
C5 is a smaller X-cap. It should be placed after the diode rectifier but before the main
capacitor filter.
XCAP should be the special class X suppression capacitor type (see Farnell Section 8
under topic Filter and Suppression pp. 508-510).X-Caps are mostly metalised polyester
and paper type.
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Notes on EMC Standards
7.2
Zainal Salam: Power Electronics and Drives
Common Mode filters
Common mode inductive filter
Power
Source
Diode Bridge
Y-CAP
Chassis
Figure 6
7.2.1 Y-CAP
Y-CAPs are used to bypass the high frequency noise from flowing to the earth. Its value is
restricted by different safety regulations, e.g.
Country
Specifications
USA
UL 478 (Industrial,
domestics)
UL 1283 (more sensitive
equipment)
BS 2135
USA
UK
Ground Leakage
Current limit
5mA; 120V/60Hz
Maximum value of
X-CAP
0.11uF
0.5-3.5mA; 120V/60Hz
0.011-0.077uF
0.25-5mA; 250V/50Hz
0.0032-0.064uF
To minimise inductor and filter size, the largest decoupling Y-CAP capacitor permitted by
the regulations should be used.
7.2.2 Common mode inductors
This is special type of inductor. It must attenuate common mode noise signal. Therefore it
must have high inductance for common mode. However it need to carry large 50Hz
current. In that sense, high inductance in not at all desirable for 50Hz.
This conflicting requirements are fulfilled by using phased inductor; i.e. two windings on
L1 are phased such that they provide maximum inductance for common mode currents but
cancelling the inductance (making it minimum) of the series mode (or the 60Hz) current.
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Notes on EMC Standards
Zainal Salam: Power Electronics and Drives
To provide the maximum inductance on the smallest core, a high-permeability core
material must be used. This is in contrary to the conventional 50Hz choke designs,
whereby low permeability core or air-gap is used. This is to prevent saturation.
The phasing effectively prevents the core from saturating for the normal 5oHZ differential
line current, as these flow in apposite phase, each winding, eliminating the 50Hz induction.
The performance of L1 for common mode is quite different. Common mode noise appears
on both supply lines at the same time with respect to the ground plane. The large shunt
YCAP helps to ensure that the noise amplitude will be the same on both lines where they
connect to the inductor. The two windings are now in phase for this condition, and both
windings behave as one, providing a large common-mode inductance. Using high
permeability ferrite toroid, the effective inductance for the common-mode inductors can be
quite high, typically several milihenrys.
Note:
EMI filters are readily available as a complete part (refer to Farnell Catalogue Section 8,
pp 492). They are, however, very expensive.
This Document is prepared by:
Dr. Zainal Salam,
UTM Skudai,
Johor Bahru.
August 2000
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